Mobile communication devices, such as cellular telephones, portable computers, personal digital assistants (PDAs), gaming devices, and the like, are configured to communicate over wireless networks. Such portable communication devices may enable communication over multiple networks, and therefore include transmitters, receivers and corresponding filters. Often, respective filters are arranged in the form of multiplexers, connecting multiple transmit and receive bands to a common antenna, enabling concurrent use of multiple radio frequency (RF) signals over various wireless networks.
The multiplexer interfaces between the antenna, or other common node, and each of the networks to enable transmitting signals on different transmit (uplink) frequencies and receiving signals on different receive (downlink) frequencies. The filters in filter branches associated with the multiplexer have to pass-through various transmitted and received signals in a defined frequency band (so-called “passband”), and at the same time block signals outside the passband (in the so-called “stopband”), especially in the in the passbands of the other filter branches used in the same multiplexer. In this manner, the likelihood of the signals of the wireless networks, passing through the respective filters, interfering with each other is greatly reduced. With the ever increasing need for mobile data, new frequency bands are being defined and enabled for the mobile communication devices. Ideally, these new frequency bands are provided in ways that the respective RF signals can be used concurrently with the presently supported wireless links, for example, by accommodating the RF signals in the new frequency bands to the same antenna(s) of mobile communication devices using multiplexers, which prevent detrimental cross-talk between the RF signals of the individual frequency bands.
From filter curve characteristics, one can differentiate between band pass filters, which provide passbands for relatively narrow frequency bands, and notch filters, which provide stopbands in relatively narrow frequency bands (passing all signals with frequencies outside these stopbands). Band pass filters and notch filters may be used in a complementary fashion, such that notch filter(s) associated with one wireless network have stopband(s) that correspond to (e.g., match) passband(s) of band pass filters(s) associated with another wireless network.
Many of the recently defined new frequency bands are at frequencies above 3 GHz, because the spectrum below 3 GHz currently is almost fully allocated. FIG. 1 is a schematic diagram showing examples of new frequency bands in the ultra-high band domain (3 GHz 10 GHz). In particular, FIG. 1 shows frequency band n77 (3.3 GHz-4.2 GHz), frequency band n78 (3.3 GHz-3.8 GHz), and frequency band n79 (4.4 GHz-5.0 GHz), together with the existing 5 GHz WiFi band (5.1 GHz-6.0 GHz). However, as shown in FIG. 1, the frequency separation of respective new frequency bands is typically low in order to maximize available bandwidth for the mobile data.
Therefore, to be effective, antenna multiplexers need to provide filtering with high bandwidths, as well as steep roll-offs, which requires low insertion loss in the respective passbands and high rejection at adjacent frequency bands. Achieving both high bandwidths and steep roll-offs is difficult with conventional filtering technologies and architectures for antenna multiplexers. Generally, technologies that provide steep roll-off, are based on resonances with a high quality factor (i.e., “high-Q”), such as acoustic wave resonances, e.g., in Bulk Acoustic Wave (BAW) or Surface Acoustic Wave (SAW) resonator devices using piezoelectric materials. “High-Q” means a quality factor (i.e., “Q-factor”) of at least 500.
However, such acoustic wave resonators typically do not support very high bandwidths required for frequency bands defined for ultra-high band domain, as the achievable bandwidth is limited by the intrinsic bandwidth of the acoustic resonance, which is defined by the material properties of the respective acoustic resonator. In contrast, other technologies that can provide very high bandwidths, e.g., filters composed of resonant circuits created from inductors (L) and capacitors (C), so called “LC circuits” or “LC filters.” However, such LC filters and/or filter components only have a low Q-factor (less than 100), and therefore may not provide sufficient roll-off steepness to protect adjacent frequency bands in the ultra-high band domain, depending on the frequency bands. Therefore, a different solution for multiplexers is needed that supports high bandwidth and steep roll-off, as well as low frequency separation, at ultra-high frequencies (3 GHz and higher).